Why does the tide rise
and fall twice a day?
You may have heard that "the Moon pulls on the ocean's water." But if that were the whole story, high tide should arrive once a day. In reality, it comes around twice a day. Let's look at the curious mechanism behind this — one where the sea even bulges out on the far side of the Earth from the Moon.
A day spent in a seaside town, from morning to evening. In the morning, a wide sandy beach stretched out below the seawall. But by early afternoon, when you come back, the sand is almost entirely underwater. By evening, it starts to reappear.
Look closely at the seawall and you'll see a band thick with shellfish and seaweed running straight and level along the wall. It marks how high the sea always rises to.
As anyone who fishes will know, this rise and fall repeats about twice a day, as regular as clockwork. "Because the Moon pulls on it" alone can't actually explain this "twice."
The mechanism: two bulges
The Moon's gravitational pull grows stronger the closer you are to it. The seawater facing the Moon is pulled more strongly than the rest of the Earth, and bulges toward the Moon. So far, this matches intuition.
In fact, the whole Earth is also being pulled by the Moon and moves along with it. The seawater on the far side is pulled the weakest, and gets left behind as the Earth moves forward, so it bulges out in the direction opposite the Moon.
In other words, the sea bulges simultaneously in two places: the side facing the Moon, and the exact opposite side. Since the Earth spins once a day beneath these bulges, every coastline passes under a bulge twice a day — that's the real story behind "high tide twice a day."
"Left behind," as a tug-of-war
Picture three people in a line, all connected by a rope and being pulled in the same direction. The person at the front is pulled hard, the one in the middle moderately, and the one at the back only weakly. What happens to the line? All three move in the same direction, but the line itself gets stretched out, front to back.
That's exactly what's happening with the Earth and its oceans. The seawater near the Moon, the body of the Earth, and the seawater far from the Moon are each pulled by the Moon with different strength. As a whole, everything is pulled toward the Moon, but because of that difference, the Earth gets stretched front-to-back along the line to the Moon. The rocky Earth barely deforms, but the seawater, free to move, responds readily to this "stretching" and forms two bulges.
While the Earth makes one full turn, the Moon also moves a little further along its orbit. So to return to "directly under the same spot on the Moon" takes one full rotation plus a bit extra. That extra bit is about 50 minutes. The time from one high tide to the next is half of 24 hours 50 minutes, or about 12 hours 25 minutes. The reason the high-tide time in tide tables creeps steadily later each day is the Moon's orbit around the Earth.
Spring and neap tides — the Sun joins in too
The Moon isn't actually the only thing stretching the seawater. The Sun does the same thing (though with less than half the Moon's strength — more on why in the final expandable section).
Around new moon and full moon, the Sun, Earth, and Moon line up, and the Sun's stretching effect lines up with the Moon's. This is when the difference between high and low tide is greatest — spring tide. Around the half-moon, the Moon and Sun sit at right angles to each other, their stretching effects cancel each other out, and the difference becomes small — neap tide.
In the Ariake Sea in Kyushu, the spring-tide range reaches about 6 m. On the Sea of Japan coast, meanwhile, it's only about 0.3 m — even though the Moon's pull acts equally on both. What creates this difference is the shape of the seabed and the bay. In shallow bays that narrow toward their inner end, incoming water has nowhere to go and piles up high; when the shape of the bay also resonates with the rhythm of the tide, the swing is thought to be amplified further, much like pushing a swing in time.
Summary
High tide comes twice a day because ① the sea on the side near the Moon is pulled strongly and bulges, and ② the sea on the far side is left behind and bulges the opposite way — two bulges form at once, on either side of the Earth. As the Earth turns once beneath them each day, our coastlines pass under a bulge twice.
The Moon isn't just "pulling" on the sea.
By pulling the whole Earth along with it, it's "stretching" the sea back and forth.
This "stretching force" has actually also reshaped the Moon itself and slowed its spin. Why the Moon always shows the same face to Earth is explained in this article.
And when this rise and fall passes through a narrow place like a strait, the difference in sea level creates fast currents and whirlpools. That mechanism is explained in Why do whirlpools form in the middle of the sea?
- Using the Japan Meteorological Agency's website or a tide-table app, note the high-tide time for a nearby sea once a day (you don't need to visit the coast)
- After a week, subtract each day's time from the next to see how much later high tide gets each day
- Also note the Moon's phase, and see whether the difference between high and low tide grows around new moon and full moon
The delay should come out to around 50 minutes. If it's not exact, that's because the shape of the bay, air pressure, and wind all play a part too. Even the pattern of the shift reflects the character of the local sea.
Want to know more? — Terms, formulas, and links to the textbook curriculumWe've labelled which level each part belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Basic Physics," "Physics," or "Basic Earth Science"
- HS+An advanced high-school "Physics" topic, or treated as a textbook sidebar
- Univ.Not covered in high school — content from university specialist courses (physical oceanography, geodesy)
- ResearchNot yet settled as "established fact" even at university — something researchers are actively investigating
MSTerms: this phenomenon has names
- Tide: the formal name for the sea's rise and fall. The risen state is called high tide, and the fallen state low tide.
- Tide-generating force: the "stretching force" mentioned in the article. It arises from the fact that the strength of the Moon's (or Sun's) pull differs from place to place — it's a force born of that difference.
- Spring tide and neap tide: the periods when the difference between high and low tide is largest and smallest, respectively. Spring tide occurs around new moon and full moon; neap tide around the half moon.
- Tide level: the height of the sea surface at a given time. The Japan Meteorological Agency continuously measures this at observation points across the country.
MSHSChecking with formulas: the tidal interval and the daily shift
Let's actually calculate and confirm the article's "about 12 hours 25 minutes" and "about 50 minutes later each day." All you need is addition, multiplication, and division.
| In symbols | T_high tide = T_moon ÷ n, where T_moon = 24 hours + Δt |
| In words | The interval between high tides = the period for the Moon to return to directly overhead, divided by the number of bulges. That period is one day plus the extra time needed to catch up with the Moon |
| Where the formula comes from | Because the Earth's rotation and the Moon's orbit go the same direction, the Earth has to turn a bit extra to "catch up" with the Moon (this is the idea of a synodic period, like a chase). There are two bulges, one facing the Moon and one opposite, so n = 2 |
| One rotation of the Earth | 24 hours |
| Period for the Moon to return "directly overhead" | about 24 hours 50 minutes (includes the extra ~50 minutes from the Moon's own orbital motion) |
| Number of ocean bulges | 2 (facing the Moon, and opposite it) |
| Period of the Moon's phases (days for the Moon to orbit once, as seen from the Sun) | about 29.5 days |
| Time for the Earth to rotate 1 degree | about 4 minutes (360 degrees in 24 hours) |
| Angle the Moon advances per day (degrees) | 360 ÷ 29.5 ≒ 12.2 |
| Extra time for the Earth to rotate that angle (minutes) | 12.2 × 4 ≒ 48.8 |
Since the Moon keeps moving a little further ahead even while the Earth is catching up, the actual Δt comes out to about 50 minutes.
| Convert 24 hours 50 minutes to minutes (the hours part) | 24 × 60 = 1440 (minutes) |
| Add the extra 50 minutes | 1440 + 50 = 1490 (minutes) |
| Halve it, since there are two bulges | 1490 ÷ 2 = 745 (minutes) |
| Convert back to hours | 745 ÷ 60 ≒ 12.4 (hours) = about 12 hours 25 minutes |
This nearly matches the interval between high tides in a real tide table. You can see that the image from Figure 1, of "passing under the two bulges in turn," translates directly into this number.
| Ariake Sea spring-tide range (approx.) | about 6 m |
| Sea of Japan coast range (approx.) | about 0.3 m |
| Ratio | 6 ÷ 0.3 = 20 (times) |
Even under the same Moon, the shape of the bay alone can create a twentyfold difference. The tide-generating force only decides "how hard the sea is being stretched." How many metres the sea surface actually moves is a joint product of that force and the shape of each particular sea.
HSHS+Why is the Moon the star, and the Sun a supporting player?
HSIn the law of universal gravitation taught in high-school physics, gravitational pull is inversely proportional to the square of the distance. The Sun is far more massive than the Moon, so the Sun's overall pull on the Earth is overwhelmingly stronger. It seems strange, then, that the Moon is the star of the tides.
HS+The key is that what generates tides isn't "the pull itself" but "the difference in pull from place to place." If you calculate the difference in pull between two points separated by Earth's diameter, you can show that this difference is inversely proportional to the cube of the distance (it comes from differentiating the inverse-square formula with respect to distance). Because closeness matters so much more when cubed, the Moon's tide-generating force ends up a little over twice the Sun's. A worked numerical comparison, done as "two divisions," appears in the expandable section of the article on the Moon's rotation.
Univ.The real ocean is far more complex than "two bulges"
The picture in Figure 1 — "two bulges circling the Earth" — is called equilibrium tide theory, and it explains the rhythm and count of tides well. But the real ocean is walled in by continents, its depth varies enormously, and the bulges can't simply travel all the way around. Actual tides instead move as waves circling around a rotating center called an amphidromic point in each ocean basin (dynamic tide theory). Amplification like that in the Ariake Sea is also handled within this framework, as a resonance between the bay's natural period and the tidal rhythm. Modern tide forecasts combine this kind of dynamic calculation with observational data.
📖 For the derivation of the formulas and further detail: Tide (Japanese Wikipedia)
ResearchWhat's still not fully understood
Tides have been studied for centuries, but parts of the picture are still being actively researched.
- Where "internal tides" in the deep sea end up. It's not just the sea surface — layers of water deep within the ocean also rise and fall with the tidal rhythm (internal tides). Where and how much these waves break and mix the seawater is considered a major unsolved problem for calculating how heat moves through the ocean and for climate modelling.
- How long was a day in the distant past? Friction from the tides has been gradually slowing the Earth's rotation. Growth bands preserved in fossil corals suggest a day was around 22 hours a few hundred million years ago, but how to interpret records from even older eras is still an active area of research.
- Oceans and tides on icy moons. On moons such as Jupiter's Europa, heat from tidal flexing is thought to keep an ocean liquid beneath the ice. Working out exactly where and how much of that heat is generated is an ongoing research question that shapes what we think these extraterrestrial oceans are like.
In other words, even this article describes things "as best understood so far." Buried within this everyday, endlessly repeating phenomenon are questions that stretch all the way from climate to the oceans of other worlds.
Links to the school curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science — the Moon's motion and phases | The shift in high-tide time; the link between spring/neap tides and the Moon's phase |
| HS | Physics — universal gravitation / Basic Earth Science — oceans | How gravity depends on distance; the basic rhythm of tides |
| HS+ | Physics — applications of universal gravitation (advanced content) | Why tide-generating force is inversely proportional to the cube of distance |
| Univ. | Physical oceanography / geodesy | Equilibrium and dynamic tide theory, amphidromic points, bay resonance |
| Research | Physical oceanography / planetary science (unresolved) | Internal tides and ocean mixing, the length of the ancient day, tidal heating on icy moons |
| — | Everyday connections | Reading a tide table, regional differences in tidal range, the band of shellfish on the seawall |
- Japan Meteorological Agency, "Knowledge of Tides and Sea Level" (気象庁「潮汐・海面水位の知識」) — the mechanism of tides, spring and neap tides, and nationwide sea-level observation.
- National Astronomical Observatory of Japan, Calendar Computation Office (国立天文台・暦計算室), explanatory material on moonrise/moonset and the tidal cycle.
- Standard physical oceanography textbook treatments of equilibrium and dynamic tide theory (e.g. Sanae Unoki (宇野木早苗), Coastal Physical Oceanography (『沿岸の海洋物理学』)).
- Munk, W. & Wunsch, C., Abyssal recipes II: energetics of tidal and wind mixing, Deep-Sea Research I 45, 1998 (internal tides and ocean mixing).
- Williams, G. E., Geological constraints on the Precambrian history of Earth's rotation, Reviews of Geophysics 38(1), 2000 (estimating the length of the ancient day).
※This article is a general-audience science explainer. The figures given are approximate, meant to aid understanding of the mechanism. Actual tide levels are also affected by air pressure and wind. When observing the coast, fishing, or gathering shellfish, always check the latest tide tables and local notices, and act with care.